Process for producing apertured body comprising casting an alloy,plastically deforming the cast alloy,and etching to remove one of alloys,and body produced thereby



Oct. 13, 1970 L ETAL 3,533,863

PROCESS FOR PRODUCING APERTURED BODY COMPRISING CASTING AN ALLOY.PLASTIGALLY DEFOBMING THE CAST ALLOY. AND ETCHING TO REMOVE ONE OFALLOYS, AND BODY PRODUCED THEREBY Filed Dec. 30, 1968 Zr ve n tor-sDdeycn Robert Lee; I I Russell,

The/r A ttor'n e y.

Unitcd States Patent 3 533 863 PROCESS FOR PRODiICliIG APERTURED BODYCOMPRISING CASTING AN ALLOY, PLASTI- CALLY DEFORMING THE CAST ALLOY, AND

ETCHING TO REMOVE ONE OF'ALLOYS, AND BODY PRODUCED THEREBY Daeyong Lee,Scotia, and Robert R. Russell, Burnt Hills,

N.Y., assignors to General Electric Company, a corporation of New YorkFiled Dec. 30, 1968, Ser. No. 787,751

- Int. Cl. C23f 1/02 US. Cl. 156-18 11 Claims ABSTRACT OF THE DISCLOSUREThe present invention relates generally to the art of producing articleshaving openings or apertures of uniquely small cross-sectionaldimension.

It has long been recognized that a thin sheet-like body having openingsof extremely small size would have a number of potentially importantuses. In the past, metal filters have been made by weaving wires to formfine screens but the resulting holes are coarse. In another method, afine metal powder is mixed with another powder which may be metal, andthe mixture is sintered to form a dense mass which is then etched toremove one of the powders. The pores of the resulting product, however,are not regular in size or shape. Porous bodies such as expanded Vycortubing and certain filter papers have holes of minimum cross-sectionaldimension, but they cannot be used for a number of applications wherehigh tensile strength, or electrical or metallic properties are desired.Although filters having holes of small cross-section have also beenprepared by irradiating a sheet of plastic and etching away theradiation tracks, this method cannot be used on metals.

By virtue of the present invention, openings or apertures can be formedin thin sheets of an alloy to produce articles for uses not met by priorart porous bodies.

Further, in accordance with this invention, apertures or recesses orsubstantially uniform size can be produced. In addition, by partially orcompletely filling these apertures or recesses, as the case may be, withselected materials, composite bodies for a wide variety of specialpurposes and uses can be made.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the figures accompanying and forming apart of the specification, in which:

FIG. 1 is a cross-sectional view magnified 1000 times of a silver-copperingot of eutectic composition cast as disclosed in Example 1 of thepresent application, showing the fine two-phase substantially lamellarstructure of the cast alloy.

FIG. 2 is a cross-sectional view magnified 1500 times of a silver-copperdisc showing the equiaxed structure produced by hot swaging the ingot ofFIG. 1 and compressing a transverse slice of the ingot as disclosed inExample 1 of the present application.

Described broadly and generally, an article of this invention is a solidbody which has a plurality of re- 3,533,863 Patented Oct. 13, 1970 icecesses or apertures of minimum cross-sectional dimensions. As usedherein, by the terms pore, aperture or hole is meant a hole extendingfrom one surface of the etched sample through the opposite surface. Onthe other hand, by the term recess is meant a hole extending from onesurface of the etched sample and ending within the etched sample. Inaddition, the word phase defines a quantity of matter havingsubstantially the same properties such as crystal structure andcomposition.

Briefly stated, the process of the present invention comprises castingan alloy which in the solid state is comprised of at least two phases inlamellar form, as for example platelike or rodlike form, that aresubstantially alternately disposed. The cast alloy is plasticallydeformed to convert the lamellar structure of the phases to phases of asubstantially equiaxed structure. The resulting alloy is etched toselectively remove at least part of one equiaxed phase to formapertures, or if desired, recesses.

The alloy of the present invention is one which in its cast solid stateis comprised of a mixture of at least two lamellar substantiallyalternately disposed phases. Specifically, there are three types ofalloys which have such a characteristic lamellar or banded phasestructure in the solid state, namely eutectic alloys, eutectoid alloysand monotectic alloys.

A eutectic alloy is one which freezes or solidifies from the liquidalloy solution to a mixture of two solid phases. Representative of suchalloys are Ag Al-Al, Ag-Cu, Ag-Ge, Ag-Pb, Al-Be, CuAl -Al, Al-Ge, Mg Al-Mg, Al-Al Ni, Al-Si, Al-Zn, Cu-Cu As, Fe-Fe As, InAs-As, Mn-Mn As,Ni-Ni As As-Pb, SnAs-As, AllgBi-Bl, AuCd -Cd, Au-CeAu Au-Ge, Au-AuSbAu-Si, Au-Tl, Au-AuZn, Ti-Ti B, Be-Be Co Cr-Be Cr, Ni-BeNi, Mg-Mg BiBi-Sn, TlBi Bi, Fe-C, Mg-Mg Ca, Ca-Ca Pb, Ca-CaTl, Cu-In, Cd-Pb, Cd-CdSb Cd-Sn, Cd-Ti, Cd-Zn, CeCu-Ce, Co-CoSn, CoTiCo Cr-ZrCr Cu-MgCu CuSb-Sb, Te-CuTe, Fe-TaFe Fe-TiFe FGZIFEZ, Ga-In, Ge-Mn Ge Sb-Ge, In-Zn,La-LaZn, Li-LiZn, Mg Sr-Sr, Ti-TiMin, Zr-ZrMo Nb-Th, ZrO -Zr, Pb-Sb,Pb-Sn, PtSi-Si, Sb-Sb Te Sb-ZnSb, Ti-Ti Si Tl-Sn, Zn-Sn, and Th-Ti.Typical of the non-metallic eutectic alloys are NaF-LiF, LiF-NaCl,NaF-NaCl and NaF-NaBr.

A eutectoid alloy is one which is a mixture of two solid phases formedduring cooling from a simple solid phase. Representitive of such alloysare Ag Al-Al, Ag-Cu, C-Fe C, MgCe-Ce, Cr-Ni, Ti-TiCr Cr-Zr, Fe-Cu,Zr-Cu, Mn-Mn ln, Ti-Mn, Zr-ZrMo and Zr-Th.

In the monotectic type of alloy, the liquid alloy forms a solid phaseduring cooling and also a liquid phase of ditferent composition which,with additional cooling, solidifies to form the second solid phase.Representative of this type of alloy are Cu-Pb, -BiU-U, Zn-Bi, Ca-Na,Cr-Cu, Cu-Cu Te, Ti-Ti Cu, Cu-Tl, Tl-P, and Zn-Pb.

The composition of the alloy of the present invention is one which uponbeing cast cools to a solid which is comprised of at least two solidlamellar substantially alternately disposed phases. 'For the eutecticalloy, this would be the eutectic composition or a composition closethereto. For the eutectoid type of alloy, this would be the eutectoidcomposition or a composition close thereto. Likewise, for the monotecticalloy, this would be the monotectic composition or a composition closethereto. In each instance, the range that the composition may vary fromthe eutectic, eutectoid or monotectic compositions is determinableempirically for the specific alloy. For a majority of alloys, such rangevaries generally up to about 10% by weight from the eutectic, eutectoidor monotectic compositions.

Generally, in carrying out the instant process, the alloy components aremelted together to obtain as uniform a molten sample as possible. Themolten sample is then cast by a conventional method to the desired size.

The specific form of the lamellar phases produced in the cast solidalloy depends upon the specific composition of the alloy and thetemperature gradient at which it is cooled. The temperature gradient mayvary widely and depends largely on the size of the lamellae desired tobe produced in a specific alloy. The greater the temperature gradient,the faster is the rate of cooling, and the finer and closer are thelamellae. Conversely, as the temperature gradient is reduced, the rateof cooling is slowed and fewer lamellae are formed but these lamellaewill be, substantially, correspondingly thicker.

The cast alloy is plastically deformed to convert the lamellar phasestructure to a substantially equiaxed structure. A number ofconventional methods can be used to carry out such deformation. Forexample, the alloy can be worked while hot and plastic by methods suchas extrusion, rolling, compression or swaging. The specific temperatureat which the alloy is hot worked depends largely on its malleability atsuch temperature. The alloy can also be cold worked, i.e., worked atroom temperature, by methods such as rolling and swaging. The size ofthe equiaxed grains initially produced in the worked alloy dependslargely on the size of the lamellae formed upon casting the alloy.Additional working, i.e. deformation, of the alloy refines the equiaxedgrains to an even finer equiaxed structure. On the other hand, heatingthe worked alloy, preferably in an inert atmosphere for those alloyswhich have a tendency to interact with gases such as oxygen, nitrogen orhydrogen, will enlarge the equiaxed grains to the desired size.

Generally, prior to etching, the alloy is cut in a direction transverseto the oriented phases to a size desired for etching. Any conventionalcutting means such as a moving saw, cut-off wheel or spark cutting canbe used. For example, for the preparation of a filter, it is slicedtransversely to a thickness depending largely on the strength of thealloy and alloy ductility in relation to the thickness desired in thefinal etched product. The slice of alloy can be etched directly, orpreferably, it is mechanically polished prior to etching to remove thedistorted surface layer generated during mechanical slicing. Suchpolishing is also useful to reduce the slice to a desired thickness,such as, for example, a foil.

The particular etchant used depends largely upon the specificcomposition of the phase to be removed as well as the remainder of theworkpiece. Such compositions are known from phase diagrams in theliterature. If the phase diagram is not available, the compositions areeasily determinable by standard metallographic procedure and x-rayanalysis. The etchant used should selectively etch the phase desired tobe removed and should not significantly affect the remainder of theworkpiece.

The etching can be carried out in a number of conventional ways. Forexample, the alloy article can be immersed in a solution of the etchantuntil the equiaxed phase to be removed is etched sufiiciently to formholes. However, if recesses rather than holes are desired, only onesurface of the workpiece should be contacted with the etchant until thephase to be removed is etched to form recesses of the desired depth. Insome instances, especially when the workpiece is as thin as a foil,electrolytic etching is preferred because it can be carried out at afast but easily controlled rate. Upon completion of the etching, theworkpiece is preferably rinsed with water or neutralizer to stop furtheretching action.

The etching procedure, whether by simple contact of the etchant with theworkpiece or other etching method, can be manipulated to obtain holes ofthe desired size. Specifically, the etched holes can be as large as thegrain size of the phase to be removed. However, holes having a sizefiner than the grain size can be produced by proper control of workpiecethickness and/or rate of chemical attack. In this respect, an importantfactor is the relative electrochemical nature of the phases,specifically their reactivity with etchants which are also electrolytes.For example, in a copper-silver alloy, the copper phase is more activethan the silver phase as shown by its position in the electromotiveseries. The copper atoms, therefore, have a greater tendency to go intosolution as ions leaving electrons on the remaining copper thus makingit negative and resulting in a galvanic effect. In the process ofetching of the alloy sample of the present invention, there appears tobe a concentration of this galvanic effect at the central portion ofeach grain being etched resulting in that area of the grain beingpreferentially attacked by etching proceeds from the center to theperiphery or boundary of the grain. This galvanic effect can beincreased by increasing the rate of etching by the use of a properelectrolyte. Advantage of this elfect can be taken in thicker samples,especially thicker foil samples, to produce fine holes since, in suchinstance, the etching can be stopped once the hole is formed and beforethe etching proceeds toward the phase boundary. On the other hand, inthinner foil samples, the etchant etches through the central portion ofthe grain at a rate too rapid to stop before it proceeds toward thegrain boundary, and therefore, produces larger holes. In addition, thefaster the rate of chemical attack, i.e. the faster the grain isdissolved by the etchant, the more difiicult the control of the etchedhole size. Therefore, the rate of attack by an etchant preferably shouldbe made directly proportional to the thickness of the foil being etched.This rate may be controlled by proper selection of the type orreactivity of the etchant and/or method of etching.

In the present invention, the specific thickness of the workpiece canvary widely and will depend somewhat on its final use. As a minimum, itneed only be thick enough to form a continuous film, generally about1000 angstroms, depending on the particular alloy used. Its minimum aswell as maximum thickness is limited by the etchability of the phasewhich serves as matrix as well as the phase to be removed. The etchedholes or recesses are of substantially uniform cross-sectional size.Their cross-sectional area, i.e. diameter, depends largely on the finaluse of the product and may be as low as about 50 angstroms or lower.There is no limit on the maximum cross-sectional area since, prior toetching, the worked alloy can be heated to enlarge the equiaxed grainsto the desired size. The present invention is especially useful forproducing porous foils.

All parts proportions or amounts used herein are by weight unlessotherwise noted.

The invention is further illustrated by the following examples.

EXAMPLE 1 An eutectic silver-copper alloy was prepared by melting 71.9percent by weight of silver and 28.1 percent by weight of copper, eachof which was about 99.999 percent pure, under argon in a graphitecrucible. The molten alloy was chill cast in a vertical aluminum oxidemold which was 1% inches in diameter and 5 inches long and which had acopper plate at the bottom. The copper plate was maintained at roomtemperature. The resulting cast cylindrical rod was about 1% inches indiameter and 1 /2 inches in height.

Metallographic examination of both ends and along the length of the rodshowed two lamellar alternatively disposed phases in the direction ofsolidification, i.e. the phases were substantially perpendicular to theplanes of both ends of the rod. The lamellar structure is illustrated inFIG. 1. The rod was cold worked, i.e. rolled at room temperature, untilit had a diameter of 0.40 inch.

The worked rod was cut transversely, i.e. in a direction transverse tothe oriented phases, by a cut-off machine to produce a sample inch indiameter and 7 inch long. This sample was heated in a resistance-typefurnace having a helium atmosphere to a temperature of 675 C. and heldat this temperature for 10 minutes. At the end of this time, while stillat a temperature of 675 C., it was compressed by means of platens to athickness of about 35 inch.

The compressed specimen looked like a disc. It was examinedmetallographically and found to have a two phase structure with eachphase being comprised of substantially equiaxed grains approximately onemicron in size, a micrograph of which is shown in FIG. 2.

The thickness of the disc was reduced by grinding to about 0.002 inchthickness. The resulting foil was polished by A1 abrasives, and finallypolished by a mechanicalchemical method with a Cr O abrasives slurry in5% aqueous CrO solution to a thickness of about 3 microns. The polishingalso produced mirror smooth surfaces.

The foil was immersed in an etching solution at room temperature forabout 15 seconds to etch out the copper phase. The silver phase wasreduced slightly in thickness to approximately 2 microns. The etchantwas comprised of 100 ml. concentrated (38%) hydrochloric acid containingone gram of chromtrioxide. The etched foil was rinsed with water andexamined.

Transmission electron micrographs of the foil showed that the copperphase had been partially etched away to produce holes. The remainder ofthe foil was not substantially affected except that the foil now had athickness of about 2 microns. About 50% of the holes had a diameter ofabout 200 angstroms. Some of the remaining holes appeared to be finer indiameter, i.e. as high as about 500 angstroms. The holes passedsubstantially straight through the foil and were substantially parallel.

The etched foil was strong and flexible. It appeared to.

be useful as a selective membrane or filter for specific applicationswhere metallic properties are desirable.

EXAMPLE 2 A magnesium-aluminum eutectic alloy was prepared by melting67.7 percent by weight magnesium and 32.3 percent by weight of aluminum,each of which was about 99.999 percent pure, under argon in a graphitecrucible. The molten alloy was chill cast in a vertical aluminum oxidemold which had a copper plate at the bottom. The copper plate wasmaintained at room temperature. The resulting ingot was 8 inches longand 2 /2 inches in diameter. It was cut to produce cylinders 2% incheslong and 2%; inches in diameter.

Metallographic examination of both ends and along the length of thecylinders showed two lamellar substantially alternately disposed phaseswhich were substantially perpendicular to the planes of both ends ofeach cylinder.

Each such cylinder was extruded at 300 C. to a rod having a diameter of/2 inch. The extrusion was carried out in a copper container with A inchwalls to minimize surface tearing problems.

A thin transverse slice, approximately 0.50 inch thick, was cut from themagnesium-aluminum rod by means of an abrasive cut-off wheel and thenground and polished with A1 0 abrasives on both faces to produce a foil.002 inch thick which was substantially free of disturbed or workedmetal. The foil showed a two phase structure with each phase beingcomprised of substantially equiaxed grains approximately one micron insize.

The foil was thinned, i.e. etched and further polished electrolytically.Specifically, the foil was made the anode in a D.C. cell at a potentialof 20 v. D.C. to a stainless steel cathode in an electrolyte solutioncomprised of 20 ml. (70%) perchloric acid and 80 ml. glacial aceticacid. The electrolyte was cooled to slightly below room temperature butmaintained above the freezing point of the glacial acetic acid andagitated. After 5 seconds, the foil was removed, washed in ethanol anddried. Its thickness had been reduced to approximately 3 microns, andthe magnesium phase was substantially removed, leaving holes in the thinfoil.

Examination of the etched foil by transmission electron microscopy andelectron micrographs of it showed that the magnesium phase had beensubstantially removed and that the holes were approximately 1 micron indiameter. The holes passed substantially straight through and weresubstantially parallel and uniform in size.

EXAMPLE 3 Additional foils of the magnesium-aluminum eutectic alloy wereprepared as disclosed in Example 2. These foils were electrothinned,i.e. etched and further polished electrolytically, as disclosed inExample 2 except that the final thickness of each etched specimen wasapproximately 5 microns.

Examination of the etched foils in the same manner as disclosed inExample 2 showed that the magnesium phase had been partially removedresulting in substantially uniform holes having an average diameter lessthan 0.5 micron. This indicates that control of the size of the holescan be achieved by varying the thickness of the foil. The etched foilsappeared to be suitable for use as filters.

Since the porous etched solid of the present invention can be producedin thin foil form having high tensile strength, it is especially usefulas a filter for the separation of very fine materials, as for example inthe purification of water. Such a filter allows good fluid flow sinceits thiness ofiters little drag or resistance for fluid to pass through.In addition, its high tensile strength would allow pressure to beapplied to the fluid to still further increase flow.

If desired, composites can be formed for a wide variety of specialapplications by filing the holes or receses of the etched material ofthe present invention with a foreign material, i.e. a material differentfrom that of the etched material. For example, they can be filled withsuperconductive material or with iron particles to produce oriented,single-domain ferromagnetic sheets.

It will be apparent to those skilled in the art that a number ofvariations are possible Without departing from the scope of theinvention.

Additional methods of treating alloys to produce a soid two phasestructure wherein one phase is distributed in a fine form in a matrixcomprised of the second or other phases and wherein said finelydistributed phase is selectively removed by etching and/or articlesformed therefrom are disclosed and claimed in the following copendingapplications:

U.S. patent application Ser. No. 787,838 (Docket RD- 2813) filed of evendate herewith in the name of Daeyong Lee and assigned to the assigneehereof is directed to the treatment of an alloy having thecharacteristic of being comprised of at least two phases in the solidstate to produce at least one phase in a fine form distributed in amatrix comprised of the second or other phases. The resulting treatedarticle is etched to remove the finely distributed phase to produceapertures, or if desired, recesses.

U.S. patent application Ser. No. 787,802 (Docket RD- 1589) filed of evendate herewith in the name of Harvey E. Cline, Robert R. Russell andWarren DeSorbo, and assigned to the assignee hereof is directed to thedirectional solidification of a eutectic alloy to-produce a structurewherein one of the phases is present as a plurality ofsubstantiallyparallel rods passing through the second or other phaseswhich serve as the matrix. The directionally solidified structure isetched to selectively remove the rod-like phase to form straight throughapertures or, if desired, recesses.

U.S. patent application Ser. No. 787,837 (Docket RD-l749) filed of evendate herewith in the name of Harvey E. Cline, Robert R. Russell, andWarren De- Sorbo, and assigned to the assignee hereof is directed to thepreparation of thin porous metallic film with substantially parallel anduniform apertures by means of a replication technique. The etchedarticle produced in the aforementioned U.S. patent application Ser. No.787,802

7 (Docket RD-1589) is used as a master from which a negative replica isformed. The negative replica is then used as a substrate on which thereis deposited metal which is then recovered from the substrate as aporous film.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A method for preparing an article with apertures or recesses whichcomprises providing a cast alloy body which in the cast solid state iscomprised of at least two phases having a substantially lamellarstructure, plastically deforming said cast alloy body to convert saidlamellar phase structure to a substantially equiaxed structure, andetching the resulting alloy article to selectively remove at least partof one phase to form recesses or holes in the article.

2. A method according to claim 1 wherein said cast alloy of the body isa eutectic type alloy.

3. A method according to claim 2 wherein said alloy body is formed fromsilver and copper.

4. A method according to claim 2 wherein said alloy body is formed frommagnesium and aluminum.

5. A method according to claim 1 wherein said alloy body is of aeutectoid type alloy.

6. A method according to claim 1 wherein said alloy body is of amonotectic type alloy.

7. A method according to claim 1 'wherein said alloy body is reduced toa foil prior to etching.

8. The product produced by the method of claim 1.

9. The product produced by the method of claim 2.

10. The product of claim 8 wherein said recesses or holes contain aforeign material.

11. The product produced by the method of claim 1 in which the aperturesare of substantially uniform diameter less than about one micronthroughout their lengths.

References Cited UNITED STATES PATENTS 2,895,819 7/1959 Fiedler 75203,236,706 2/1966 Kucher 156-18 3,364,017 1/1968 Kirkpatrick 15618 X JOHNT. GOOLKASIAN, Primary Examiner J C. GIL, Assistant Examiner US. Cl.X.R.

